Solid state imaging devices, including charge coupled devices (CCD), complementary metal oxide semiconductor (CMOS) imaging devices, and others, have been used in photo imaging applications. A solid state imaging device circuit includes a focal plane array of pixels as an image sensor, each pixel including a photosensor, which may be a photogate, a photoconductor, a photodiode, or other photosensor having a doped region for accumulating photo-generated charge. For CMOS imaging devices, each pixel has a charge storage region, formed on or in the substrate, which is connected to the gate of an output transistor that is part of a readout circuit. The charge storage region may be constructed as a floating diffusion region. In some CMOS imaging devices, each pixel may further include at least one electronic device such as a transistor for transferring charge from the photosensor to the storage region and one device, also typically a transistor, for resetting the storage region to a predetermined charge level.
In a CMOS imaging device, the active elements of a pixel perform the necessary functions of: (1) photon to charge conversion; (2) accumulation of image charge; (3) resetting the storage region to a known state; (4) transfer of charge to the storage region; (5) selection of a pixel for readout; and (6) output and amplification of a signal representing pixel charge. Photo charge may be amplified when it moves from the initial charge accumulation region to the storage region. The charge at the storage region is typically converted to a pixel output voltage by a source follower output transistor.
CMOS imaging devices of the type discussed above are generally known as discussed, for example, in U.S. Pat. No. 6,140,630, U.S. Pat. No. 6,376,868, U.S. Pat. No. 6,310,366, U.S. Pat. No. 6,326,652, U.S. Pat. No. 6,204,524, and U.S. Pat. No. 6,333,205, assigned to Micron Technology, Inc.
A typical four transistor (4T) CMOS image sensor pixel 100 is shown in FIG. 1. The pixel 100 includes a photosensor 102 (e.g., photodiode, photogate, etc.), transfer gate 104, floating diffusion region FD, reset transistor 106, source follower transistor 110, and row select transistor 112. The photosensor 102 is connected to the floating diffusion region FD by the transfer gate 104 when the transfer gate 104 is activated by a transfer control signal TX.
The reset transistor 106 is connected between the floating diffusion region FD and a voltage supply line 206. A reset control signal RST is used to activate the reset transistor 106, which resets the floating diffusion region FD to the voltage supply line 206 level as is known in the art.
The source follower transistor 110 is connected to the floating diffusion region FD and is connected between the voltage supply line 206 and the row select transistor 112. The source follower transistor 110 converts the charge stored at the floating diffusion region FD into an electrical output voltage signal VOUT. The row select transistor 112 is controllable by a row select signal ROW for selectively connecting the source follower transistor 110 and its output voltage signal VOUT to a column line of a pixel array.
In order to capture images with greater resolution while also maintaining a small image sensor, it is desirable to design image sensors with a large number of relatively small pixels. As pixels become smaller, however, many of the transistors responsible for reading out the pixel signal, such as transistors 106, 110, and 112 in FIG. 1, cannot be made smaller and begin to take up most of the space allocated to each pixel 100. Consequently, the photosensor 102 becomes smaller while more of the pixel area is used by the pixel transistors, such that, the pixel's 100 fill factor, which is the percentage of a pixel that is photosensitive, is reduced. As photosensor size and pixel fill factor shrink, the amount of light that is converted to a signal within each pixel decreases as well. Another problem relates to the metal routing layers used to convey signals e.g., control signals, for the readout circuits between the various transistors of the pixel. As pixels are made smaller, these metal routing layers, which are located in metal layers above the pixel, become more obstructive to light that would otherwise reach the photosensor 102. Accordingly, there is a need for a pixel architecture that allows for smaller pixels with higher fill factors.